Page:Popular Science Monthly Volume 5.djvu/484

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468
THE POPULAR SCIENCE MONTHLY.

adhesion of surfaces confined to a single leaf, as in the iris. But these adhesions are all topical, if I may use the word, and do not affect the type and general axis.

Now, this involution, by which the progress of evolution takes place, may, as remarked, and strange to say, be described as a failure of development—a failure to unfold into the original type, constituting, by the very failure, a new and higher type. If we watch the development of any leaf from venation, we observe that in some way it was folded down upon itself, or rolled up with its fellows of the same bud, and before coming to adult age it expanded into the recognized form of its order. Let but that expansion fail, and the evolution may be said to fail. A happy failure; for to this we owe the production of grain and fruit—the food of man. Adhesions take place within the buds, which change the leaf-buds into flower-buds. The change is always made in the embryo, while still plastic and capable of being moulded into new forms.

It would be delightful to follow the immortal Linnæus, Wolf, Goethe, and the grand army of enlightened living botanists, who have illustrated this beautiful transcendental history of leaves, and flowers, and fruits; but time forbids at present. Suffice it to say that, so far as the vegetal kingdom is concerned, the doctrines of involution have become the common property of the scientific world.

Let us pass on to the so-called animal kingdom.

And first, as to the bridge we cross. True, it is somewhat the fashion to tell us that here is a great gulf fixed, and no crossing was ever possible. Yet Nature found a bridge somewhere, and we ought not to despair of finding at least some remains of the abutments.

To illustrate this passage from the vegetal to the animal plan of structure, take a hollow India-rubber ball, which may very well represent a cell enlarged a few thousand diameters. It is a perfect image of an external living creature, and is also typical of one stage of the development of the ova of the lower orders of animals—probably of all animals. But it is the adult form of plants. The vital functions of such a cell are all within; and there is no communication with the external world, except by osmotic action. Through this same cell wall—this same external coat—and by means of it, nutrition and aeration are both carried on. This is an exothen.

Now, by pressing the finger upon a part of the elastic coat, a portion of it sinks in, and you have a cup—a cup with double walls and a space between them. This is an involution; and by means of such an involution as this Nature transforms a plant into an animal.

Recently-published observations of Kowalewsky declare that he has seen this sort of transformation actually take place in the growth of the embryo of a creature as high in the scale as the Ascidians. Doubtless, in Nature, the true process was a failure at one point to fill out the rounded fullness of the ball; some contraction, some atrophy,